In drilling oil and gas wells, it is important that geologists and engineers have as much knowledge as possible of the characteristics of the earth formations through which the well passes. Such knowledge is useful for effectively completing a well in a borehole which has been drilled, and is exceedingly helpful in determining the overall characteristics of hydrocarbon producing formations for planning the drilling of additional adjacent wells. For these reasons a large industry has emerged for performing the services of well logging.
The process of providing information as to conditions in boreholes in the earth can be accomplished while drilling by transmitting information up the borehole such as revealed in U.S. Pat. No. 3,964,556, issued to Marvin Gearhart et al, entitled: "Downhole Signalling System". While this method of conveying downhole information while drilling is extremely important and gaining in acceptance by the petroleum industry, nevertheless, the most common means of providing downhole information is practiced by lowering tools in a borehole by means of a cable extending from the earth's surface.
When a well to be logged is vertical, or nearly vertical, logging tools can be effectively run in the borehole by means of a cable since the weight of the tools and the weight of the cable is sufficient to overcome any friction of the tool and cable against the borehole wall. However, difficulty is experienced if the borehole is inclined relative to the vertical. In recent years more and more boreholes are drilled wherein at least a portion of the borehole is at an angle relative to the vertical. This is especially true in drilling boreholes from offshore drilling platforms wherein it is desirable to drill as many wells as possible from a single platform location and in which it is important that the producing formation be penetrated at distances as far as possible from the location of the platform.
When boreholes are inclined relative to the vertical, impediments to the passage of a logging tool suspended by a cable can seriously interfere when such impediments would normally be of no concern if the borehole is vertical or near vertical. In drilling through various formations, the diameter of the borehole may be enlarged for short lengths due to erosion by the drilling fluid. These enlargements in the diameter of the borehole can cause recesses or pockets in inclined boreholes which tend to trap a tool suspended on a cable when the tool is being lowered by gravity into the borehole.
When an operator is unable to cause a logging tool suspended on a cable from going to the bottom of the borehole, a serious problem develops. In present practice, the only practical alternative is for the logging tool to be pushed to the bottom by means of a drill string. This is time consuming, expensive, and, in addition, the drill string tends to damage the logging tool cable since the cable and drill string must be inserted into the borehole simultaneously.
For these reasons it is highly desirable that a more effective method of moving a logging tool to the bottom of a borehole for well logging be devised. In attempting to overcome these problems, others have provided apparatus intending to pull a cable supported logging tool in a borehole. As an example of the effort of others, reference may be had to the following U.S. Pat. Nos.:
______________________________________ 2,776,564 4,192,380 3,554,284 4,166,500 3,550,684 4,168,747 4,082,144 3,036,530 4,064,939 2,650,314 4,282,523 ______________________________________
These patents show various means of moving a logging tool in a borehole, such as in U.S. Pat. No. 2,650,314 which teaches the use of an electric motor supplied by power from a cable extending from the earth's surface, the electric motor rotating propeller type blades moves well fluid past the tool imparting a thrust to pull the cable with it. A more recent example of efforts to pull a cable in a borehole is illustrated in U.S. Pat. No. 4,113,236. This tool utilizes an internal pump. Fluid is drawn into the interior of the device and moved by an electrically operated pump to be expelled through fluid outlets in the rear of the device to provide a thrust to pull the cable. However, the prior art has not been applied successfully on a large scale commercial basis in the petroleum industry because of problems and limitations in the devices illustrated in these issued patents. The present disclosure, in addition to revealing improvements in the concepts exemplified by these prior issued patents, provides basic departures from known apparatus and techniques for moving well logging equipment down an inclined borehole.